This invention relates to a control circuit that controls the application of a high frequency alternating voltage signal for power supply and gas discharge lamp operations with active power factor corrections.
It is a problem in the field of electronic power supplies and gas discharge lamp ballasts to produce an inexpensive and simple control circuit that provides all control functions including active power factor corrections. The typical architecture of an electronic gas discharge lamp circuit is such that a high frequency alternating current is used to power the circuit. The low frequency 50/60 Hz input alternating power source is first converted into a DC power by a full wave rectifier. This DC power source is then converted into to a high frequency alternating power source, usually higher than 20 kHz, to provide the power to the gas discharge lamp.
After full wave rectification, In order to reduce the variations in the DC voltage, often a large smoothing capacitor is used. The current drawn by the smoothing capacitor causes harmonic distortions in the input AC line at times when the smoothing capacitor is charging. The charging time of the smoothing capacitor is very small if a large smoothing capacitor is used and all the required charge is loaded into the smoothing capacitor in a short period of time. This rapid charging of the smoothing capacitor at the peaks of AC sinusoids is the cause for harmonic distortions and low power factor.
A control circuit that controls the operation of the gas discharge lamp operation may also be used for active power factor corrections. The gas discharge lamp control circuit of U.S. Pat. No. 4,511,823 addresses these problems and provides methods for active power factor corrections, but in doing so generates a significant amount of Electro Magnetic Interference (EMI) and feeds this interference back to the input power line. The Electro Magnetic Interference is due to the use of part of the resonant circuit energy for active power corrections. By adding a large inductor to this control circuit, the interference problem can be limited, but this adds a significant additional cost and space. Thus, this solution is not cost effective, in particular, given the cost sensitivity of gas discharge lamp ballasts.
The above described problems are solved and a technical advance achieved by the present control circuit that provides an improved method for power factor correction characteristics and low Electro Magnetic noise. This new control circuit uses an Electro Magnetic Interference abatement circuit that consists of a series connected diode in one of the DC input lines from the full wave rectifier and a capacitor connected across the DC input lines from the full wave rectifier to eliminate the Electro Magnetic Interference generated by the power factor and gas lamp control circuits. This is accomplished in part by the operation of the series connected diode which blocks reverse currents, thereby preventing high frequency current present in the electronic device from flowing back to the AC input line through the full wave rectifier. In addition, the use of the capacitor across the DC input line helps to absorb high frequency current that is present on the input lines from the full wave rectifier. The cost of these two elements is small compared to the use of an inductor, yet their synergistic effect on the input lines provides a significant abatement of the Electro Magnetic Interference generated by the power factor correction and gas discharge lamp control circuits.